1. Technical Field
The present invention generally relates to semipermeable polymer membranes and methods for making same. The invention pertains still more particularly to semipermeable polyamide membranes having enhanced stability in the presence of strong oxidizers, such as chlorine, hydrogen peroxide, and concentrated and reactive metals and metallic particles; high water flux; enhanced thermal stability; and extended shelf life.
2. Background of the Invention
Reverse osmosis (hereinafter RO) utilizes a semipermeable membrane to separate water from particulate matter, colloids, dissolved salts, organics and other materials in solution or suspension. This is accomplished by feeding pressurized impure carrier water to one side of the RO membrane that allows only water molecules to pass. RO and NF are closely related selective membrane separation processes based on differences in solute-solvent permeabilities (solution-diffusion transport mechanisms) through a dense semipermeable barrier. The semipermeable barrier is typically a very thin polymer film deposited onto a microporous substrate, such as an ultrafiltration membrane, for mechanical support as illustrated. This composite architecture is the basis for the majority of commercial RO and NF membranes employed for water purification, which are produced as flat sheets for spiral wound filter modules.
Osmosis occurs when a semipermeable membrane is placed between two compartments, one containing pure solvent (water) and the other containing a solute solution (brine) with the solvent passing through the membrane to the solute-solution side. Transport occurs due to the chemical potential driving force that is caused by the presence of the solute. The exact pressure that must be applied to the solute-solution side to stop solvent flux is called the osmotic pressure. In RO, a hydrostatic pressure greater than the osmotic pressure is applied to the solute solution to reverse the osmotic flow and drive solvent back to the pure solvent side. For seawater this is near 848 ft of H2O (368 psi, 25.4 bar). In RO processes, high pressures are required to drive the solvent (water) through the membrane at an acceptably high flux rate to provide good selectivity. The water flux is proportional to applied pressure whereas the salt flux is independent of pressure.
The ability of RO filtration to produce potable water from seawater and brackish waters for a modest price is unequaled. RO treatment costs, however, remain approximately 100-1000 times higher than conventional municipal water and waste-water treatment plants. The quality of the product water in the RO process is highly dependent on the type of membrane used. RO membranes can be grouped into three main categories:                (1) Seawater membranes (3-5 wt % salt solutions operating at 800-1000 psi)        (2) Brackish water membranes (2000-10000 ppm salt solutions operating at 200-400 psi)        (3) Low-pressure nanofiltration membranes (200-500 ppm salt solutions operating at 100-150 psi)        
Similar membrane materials can be used in each category by tailoring the membrane preparation procedure and system design. RO membranes generally reject ions and small molecules greater than 2-15 angstroms in size with 99+% selectivity, however, high pressures are required depending on the application. NF membranes generally reject larger ions and small molecules greater than 10-50 angstroms in size and have much greater water permeabilities allowing for lower pressure operation and higher permeate flux. Salt rejections are typically tailored by attaching anionically or cationically charged groups (such as carboxylic acids or amines) onto the polymer backbone. The charged NF membrane creates Donnan exclusion effects resulting in high rejection (95-99+%) of divalent anions or cations, respectively, while monovalent ions are only modestly rejected (20-80%).
Semipermeable membranes used primarily in reverse osmosis (RO) and nanofiltration (NF) separations applications comprise a polymeric thin film (0.3-3 microns) semipermeable barrier layer (provides salt rejection) deposited onto a relatively thick (>50 microns) microporous substrate (provides mechanical support). This creates a thin film composite membrane architecture. The semipermeable thin film barrier layer is typically either cellulose based or polyamide based depending on the application. Polyamide based membranes have very good salt rejections (>99.5% typical for RO), are tolerant to degradation by microbes and extreme pH levels and can withstand exposure to strong solvents, but have low tolerance to strong oxidizers. Cellulose based membranes have good salt rejections (97-99% typical for RO), and better tolerance to oxidizers but are readily metabolized by microbes, or dissolved by strong organic solvents. Therefore polyamide based RO and NF membranes are typically required for separations applications in which the best salt rejections are required, the feed stream is biologically active, or extreme pH levels or organic solvents may be encountered.
As the demand rises for purifying contaminated surface water sources for consumption, reclamation and recycling of wastewater, and desalinating brackish and salt water sources, improvements in membrane performance are needed.
The most significant issues to address for reducing the cost of desalination and membrane filtration processes are to: increase membrane tolerance to chlorine and other strong oxidizers; prevent membrane fouling due to suspended particulates, mineral scaling, and biological growth; increase water permeability for higher production and recovery rates; and increase mechanical stability of composite membranes.
Currently, polyamide RO membranes require free chlorine levels to be less than 0.1 mg/L (ppm) in concentration for continuous contact without degrading the performance of the membrane. This is typically near 1000 ppm·h to 2000 ppm·h worth of total chlorine exposure tolerated by a membrane before a decline in salt rejection will occur (exposure to 1 ppm Cl2, for 1 hour is 1 ppm-h; ppm=mg/L). Increasing the oxidative stability of RO and NF membranes so that they can tolerate continuous contact with chlorine and other strong oxidizers will result in substantial savings by reducing filter element replacement frequency and feed water pre-treatment, and increasing membrane lifetime. Increasing membrane chlorine tolerance will allow more frequent cleaning/disinfection treatments to mitigate biofouling without degradation. However, efforts over the past three decades have not resulted in significant improvements on chlorine tolerance for high rejection membranes while thermal disinfection of RO membranes is rarely an option due to modest thermal stability.
For example, the Army's ROWPUs are designed to treat any water source encountered in the world at production rates of 200 to 3000 gallons per hour; therefore RO membranes used in these systems need to be effective under a wide variety of feedwater conditions. ROWPUs incorporate commercially available polyamide spiral wound RO filter elements for removal of contaminants from water. Disinfection standards for ROWPU-purified water require a 2.0 ppm (ppm=mg/L) free available chlorine (FAC) residual after a 30 minute contact time. This level of chlorine is typically added after the filtration process to avoid excessive RO membrane damage due to chlorine degradation that results in frequent element replacement. However, the addition of chlorine as a disinfectant prior to the filtration process will prevent biofouling of the RO membrane, presently one of the most challenging and costly issues in RO element longevity. Commercially available composite membranes can tolerate very little chlorine in feed water for extended periods of time (<0.1 ppm).
Desalination membranes must be used with feed-water halogen (Cl2, Br2) levels at less than 0.1 ppm, thus requiring extensive water pre-treatment including pre-filtering and charcoal bed clarification to remove oxidizers such as halogens. For seawater desalination the feedwater pre-treatment costs and membrane replacement represent up to ˜60% of the total desalination costs, around $1-$5/1000 gallons. Energy consumption for pumping, amortization and retentate treatment constitute most of the balance in cost.
Filter membrane fouling occurs when suspended solids collect, dissolved solids precipitate, or microbes attach and grow on a membrane surface leading to a decline in membrane performance. These foulants can be cleaned from RO and NF membranes by the use of flushing solutions containing cleansers and disinfectants. For example, acidic, caustic, and enzymatic cleansers often containing surfactants will remove various types of deposited organic solids or mineral scale while microbial growth can be eliminated by disinfectants such as chlorine or hydrogen peroxide.
Another problematic issue for thin film composite membranes used for RO and NF applications is delamination of the polyamide film from the porous substrate. Intermittent filter use that results in membrane pressurization/depressurization cycles as well as back-flushing promotes delamination causing filter failure. Large desalination plants are run continuously to extend membrane lifetime to 2-3 years. A standard 8 in. diameter by 40 in. long spiral-wound RO filter element costs about $790 each, or ˜$3/ft2, equating to $750,000 in membranes for a 6 million gallon per day desalination plant. In contrast, the Army's reverse osmosis water purification units (ROWPUs) have an average membrane lifetime of about 400 h, or about $9500 in RO membrane replacement every 1.2 million gallons of water produced for a 3000 gph ROWPU. Any increase in filter longevity or output will improve cost-effectiveness for filtration processes.
Typically, wetting agents are used to facilitate penetration of the thin film barrier layer into the pores of the substrate. The penetration of the polyamide film into the pores mechanically anchors the thin film in place. This mechanical anchoring, however, can weaken and fail after a filter membrane has compressed and compacted during use. It has also been found, however, that the use of wetting agents such as surfactants often results in reduced performance of the polyamide membrane. Increasing the bonding strength between the microporous substrate and the thin film barrier layer will provide longer filter element lifetimes, especially for systems used intermittently. Mechanical failure or delamination of membrane layers is another source of RO element attrition resulting from intermittent system operation. Typically, membrane replacement accounts for some 18% of desalination water treatment costs for large filter plants and can run as much as 30% for smaller units such as ROWPUs.
Polyamide-based semipermeable filter membranes are typically preferred over cellulose-based semipermeable filter membranes (cellulose acetate, cellulose acetate/nitrate blends) due to greater microbial stability and typically greater salt rejection performance. Cellulose-based membranes are susceptible to being metabolized and broken down by microbial growth. Polyamide membranes are generally fairly resistant to biological attack that leads to biofouling. They can be periodically cleaned by shock disinfection with chlorine, hydrogen peroxide or peracetic acid to minimize biofouling as well. Currently, polyamide-based membranes have very limited tolerance to chlorine (1000-2000 ppm·h) and this is the most significant drawback to general use.
The development of an all-aromatic crosslinked polyamide by Cadotte and FILMTEC has led to one of the most widely produced interfacial composite membrane types known as SW-30 that is useful in seawater and brackish water purification and is FDA-approved for food processing. The Dow Liquid Separations business manufactures the FILMTEC membranes. Spiral wound RO filter elements produced with these membranes have salt rejections of >99%. This polyamide RO membrane is based on the composition of meta-phenylenediamine (also known as 1,3-diaminobenzene; hereinafter MPD) crosslinked with trimesoyl acid chloride (also known as 1,3,5-tribenzoyltrichloride; hereinafter TAC). This polymer suffers from degradative chlorine sensitivity due to the presence of secondary amide linkages as well as aromatic ring positions that are activated toward chlorine attack by the amine/amide groups. Each of these sites susceptible to chemical attack by chlorine and other strong oxidizers leads to degradative amide bond cleavage or alteration of the aromatic ring polymer backbone structure.
Nearly all polyamide-based semipermeable filter membranes are based on the conventional polymer produced by reacting TAC with MPD and/or piperazine. Films composed of this polymer are deposited onto a microporous substrate by the interfacial polymerization method. Addition of amines or phosphates that act as acid scavengers, pH buffers, are typically used in an interfacial polymerization process to promote polymerization of polyamide polymers and achieve the performance required for various filtration applications including RO and NF. In many cases these amines and/or phosphates become incorporated into the resulting polymer to enhance membrane performance.
In the most general sense membrane preparation is a two step deposition and polymerization process. In the first step the amine starting material (monomer) is deposited onto a microporous substrate as a “liquid” or “hydrated” layer from a solution containing other amines, phosphates, and typically water as the primary solvent. The second step involves contacting the wet amine-coated substrate with a solution of TAC in an organic solvent that reacts with the amine at the contact interface (‘interfacial polymerization’) to produce a highly crosslinked polyamide/polyimide thin film membrane on the surface of the microporous substrate. Several types of reactive polyfunctional amines, acyl halides, alkyl halides, sulfonyl halides, or isocyanates can be incorporated into these films to increase membrane flux and/or rejection rates. Complexing agents such as phosphates are also incorporated to improve membrane flux and/or rejection rates. None of these approaches directly address increasing membrane stability to chlorine and other strong oxidizers. The general architecture of such a membrane may further comprise a reinforcing fabric support that is included for mechanical stability. Thin films of semipermeable polymers, such as polyamide, are formed on finely porous highly water-permeable support membranes in sheets for use in spiral wound filter elements.
Polyamide membranes based on TAC and MPD suffer from the aforementioned chlorine degradation sensitivity due to the presence of secondary amide linkages as well as aromatic ring positions that are activated toward chlorine attack by the amine/amide groups' nitrogen directly attached to the aromatic ring. Each of these reactive sites within the polymer film is susceptible to chemical attack by chlorine and other strong oxidizers leading to degradative amide bond cleavage, alteration of the polymer backbone structure or packing density, or alteration of the ionic charge distribution of the membrane.
Several studies on the chlorine degradation pathways have been reported in the art. The main results of these reported studies show that secondary amide linkages adjacent to aromatic groups allow irreversible chlorination and C—N bond cleavage of the polyamide backbone to occur. Replacement of the secondary amide proton by substitution with a tertiary amide eliminates the most facile pathway of chlorine attack and subsequent polymer breakdown. A higher degree of crosslinking also increases the polymer's resistance to chlorination by creating a larger number of bonds and creating steric hindrance that blocks reactive sites from chemical attack. Increasing water flux has been correlated with the presence of atoms with electron lone pairs that allow for better hydrogen bonding between the polymer and water. Increasing water flux has also been correlated with the incorporation of five- and six-member aliphatic ring structures that are thought to decrease the packing efficiency of the aromatic polymer backbone, which creates water-permeable microchannels in the polymer. Additives such as phenols are also known to enhance water flux due to favorable hydrophilicity.
As mentioned above, another approach to increase membrane stability is to increase the number of crosslinking bonds within the polymer structure, which can increase the steric hindrance to oxidative attack of the polyamide linkages. The current inventors examined this approach by replacing 1,3,5-tribenzoyltrichloride (three potential crosslinking sites) with all-trans-1,2,3,4-cyclopentanetetracarboxylic acid chloride (four potential crosslinking sites). This approach, however, does not reduce the number of secondary amide linkages or nitrogen-activated aromatic ring positions. U.S. Pat. No. 5,254,261 describes the use of cycloaliphatic acyl halides including 1-cis, 2-trans, 3-cis, 4-trans-cyclopentane tetracarboxylic acid chloride, 1-cis, 2-trans, 3-cis, 4-trans-cyclobutane tetracarboxylic acid chloride and 1-cis, 2-trans, 4-cis-cyclopentane tricarboxylic acid chloride.
Many other semipermeable membrane materials are known that are not amenable to the conventional fabrication process and fabrication infrastructure. To be viable, polyamide formulations must be commercially competitive and provide equivalent or superior performance characteristics over current state-of-the-art membrane materials. Semipermeable membrane materials known to exhibit better than industry standard performance either utilize cost prohibitive starting materials or do not possess large enough advantages to justify retooling for production thereof.
Increasing water permeability without decreasing salt permeability for RO membranes is also a great challenge, the current standards being on the order of 20-45 L/h/m2 or 5-10% recovery for seawater desalination.
Many environmental remediation operations require treatment of water to remove contaminants that are incompatible with current membrane technology. Strong oxidizers such as chromium(VI), permanganate, many common heavy metals (e.g., nickel, copper, silver) present severe challenges for common membrane separations processes. Ion-exchange resins are typically used for capturing these types of materials, however the resins are a consumable that must be replenished (˜$2.00/lb) or regenerated creating an additional waste stream. Reducing the treatment burden on exchange resins or eliminating use of such ion-exchange resins will help reduce consumable costs in remediation activities.
Consumer house-hold use of RO filtration for water softening or de-ionization can be made more practical and affordable by combining greater chlorine stability and increasing water permeation at low pressures. Chlorine stable membranes will allow filter pre-treatment equipment costs to be reduced, which is both an up-front cost savings and a cartridge replacement/maintenance cost savings. Reducing the operating pressure requirements for treated water production will also reduce system size/cost and reduce energy consumption. These general cost-saving features will benefit RO water purification activities in general, from house-hold consumer systems to municipal treatment operations.
Accordingly, there is a need for high performance polyamide thin film composite (TFC) membranes that are chlorine-tolerant, resistant to biofouling, and mechanically robust when used, for example, in reverse osmosis water purification systems and intermittently operated. Higher water fluxes at lower operating pressures are also desirable. Successful development of membranes with superior chlorine resistance and mechanical stability can save tens of millions of dollars each year by significantly reducing the field distribution logistics, equipment down-time, maintenance time, and costs associated with water pre- and post-treatment and frequent replacement of degraded and fouled membranes.